The present invention relates to the field of fluid and soil sampling methods and apparatus. Modern industries produce contaminants which are often released onto land. The contaminants migrate downward into the subsurface creating potential health risks. Contaminant remediation plans are implemented to remove soil and ground water contamination.
Designing a remediation plan typically requires collecting soil and fluid samples to determine the extent of subsurface contamination. The term fluid as used herein refers to both gas and liquid. Soil samples provide subsurface data including contaminant concentration for inorganic and organic compounds, grain size, mineral composition, texture, density, permeability and porosity. Fluid samples are analyzed to determine contaminant concentration, organic chemistry in the case of soil gas, and both organic and inorganic chemistry in the case of liquid.
A conventional method of retrieving a fluid sample is described in U.S. Pat. No. 4,669,554 to Cordry. The sampling device includes a sample chamber which has two check valves. One of the check valves permits groundwater to enter the sample chamber and one of the check valves permits ground water to exit the sample chamber. Groundwater enters the sampling device through a tube having openings therethrough. In use, the device is driven into the subsurface to the desired sampling depth. When the sampling device reaches the desired sampling depth, the device is pulled toward the uphole end so that the body separates from the cone assembly and the tube is exposed. Groundwater then passes through the tube and check valve and into the sample chamber. The device is then pulled out of the ground with the groundwater sample being retained in the sample chamber.
A problem which occurs when using the sampling device disclosed in U.S. Pat. No. 4,669,554, and other conventional groundwater sampling devices is that cross-contamination of the groundwater sample may occur. When the groundwater sample is collected, groundwater enters the sample chamber under the potentiometric head in the formation. When the device is withdrawn from the subsurface, the tube is still exposed and groundwater from different vertical depths may enter the tube and displace the groundwater sample from the desired depth. The check valve is not adapted to stop groundwater flow into the chamber when the device is removed from the ground.
The problem of cross-contamination of the groundwater sample is particularly troublesome when the sampling device is removed from the subsurface since the fluid in the borehole may replace the collected sample. The borehole is normally filled with turbid water which contains suspended solids or drilling mud which increase the specific gravity of the fluid in the borehole. The high specific density borehole fluids may, therefore, pass through the check valve and into the fluid collecting portion thereby contaminating the fluid sample.